A typical compressor based refrigeration system is a closed fluid circuit through which a refrigerant flows, changing from liquid to gaseous state and back, absorbing and giving up heat during the process. Such a typical system utilizes a compressor in series with a condenser, expansion valve, and evaporator. As is known, the system operates as gaseous refrigerant is compressed by the compressor and is passed to the condenser. Within the condenser the highly pressurized refrigerant gives up heat, typically to the outside environment, as it condenses to liquid form. This condensed liquid refrigerant next passes through an expansion valve which serves as a refrigerant flow control device. This expansion valve allows the compressed liquid refrigerant to experience a sudden drop in pressure causing it to cool and expand, returning to a gaseous state as it absorbs heat as it flows through the evaporator. A typical refrigeration system includes a fan within the environment to be cooled which blows the ambient air across the evaporator where it is cooled. In such a typical system, the compressor can work at a fairly constant level which allows the use of a fixed orifice expansion valve to maintain a fairly constant refrigerant flow therethrough.
While such typical refrigeration systems may operate at a fairly constant level, some application installations for refrigeration systems demand that they operate under widely varying conditions. One such application is for a refrigeration system installed in a motor vehicle. In such an application, the compressor is typically driven by a belt coupled to the motor vehicle's engine. Since the motor vehicle's engine speed differs significantly over the course of its operation, the drive input to the compressor also varies resulting in a change in the overall performance of the refrigeration system itself. In addition to the change in the system performance caused by the differing drive input to the compressor, the amount of refrigerant cooling provided by the air flow directed across the condenser also varies as the motor vehicle is driven at highway speeds or is stopped in rush hour traffic. This too alters the refrigeration system's ability to operate in this environment.
Despite these significant changes in the operating characteristics of a motor vehicle's refrigeration system, past systems still utilized a fixed orifice expansion valve to control the refrigerant flow through the system. A typical orifice size for this fixed or single stage expansion valve is 60 thousandths of an inch. In such a system, when the motor vehicle is driven at highway speeds the air conditioning system is able to provide very cold air to the passenger compartment. However, when the vehicle enters the city, the air coming from the air conditioning vents no longer feels very cold and may, in fact, be warmer than the air within the passenger compartment.
To overcome this problem, the assignee of the instant invention invented and patented a REFRIGERATION SYSTEM FLOW CONTROL EXPANSION VALVE, U.S. Pat. No. 5,715,704 which issued to Cholkeri et al. on Feb. 10, 1998, the teachings and disclosure of which are hereby incorporated by reference. The invention of Cholkeri et al. '704 is a two-stage expansion valve capable of providing differing flow therethrough based upon system conditions. The expansion valve of Cholkeri et al. '704 includes a valve body having a valve inlet for accepting refrigerant and a valve outlet for delivering refrigerant that has passed through the valve body. The valve body includes a metering head that defines first and second passageways for fluid passing through the valve body. A valving element is mounted for controlled movement within the valve body and includes a valve element passageway which, in combination with the first and the second passageways in the metering head, conveys refrigerant through the valve body to the valve outlet. A valve actuator mounted to the valve body moves the valving element to a position for restricting refrigerant flow through one passageway of the first and second passageways while allowing refrigerant to flow through the other of the first and second passageways. The valve actuator includes a control input responsive to an external control signal to control positioning of the valving element itself.
The nature of the signal at the control input of the expansion valve of Cholkeri et al. '704 depends upon the manner in which refrigeration flow is regulated. In certain applications it is sufficient to regulate flow between high and low flow rates at periodic intervals based upon a monitored parameter. For a motor vehicle, for example, the monitored parameter could be engine speed, motor vehicle speed, or compressor head pressure. Any of these going below a threshold could be used to control refrigerant flow rates. In other applications a controlled frequency pulse width modulated signal could be applied at the control input. In such an application a greater control over refrigerant flow is provided by controlling the duty cycle of the control signal, which maintains a greater degree of control over refrigerant flow.
In the exemplary application of a motor vehicle system, the two-stage expansion valve of Cholkeri et al. '704 operates in a de-energized state which corresponds to a high refrigerant flow through the first and second passageways of the metering head to provide optimum cooling during highway operation conditions. When the motor vehicle operates within the city in stop and go traffic, the expansion valve of Cholkeri et al. '704 is energized to prevent refrigerant flow through one of the first and second passages resulting in a reduced refrigerant flow therethrough. It has been found by the assignee of the instant invention that such reduced flow optimizes the cooling capability of the refrigeration system under these conditions. As an example, the high flow or de-energized state could provide an effective 60 thousandths of an inch diameter flowpath for the refrigerant. In the energized or low flow state the effective orifice size could be reduced to 40 thousandths of an inch, as an example.
While the two-stage valve of Cholkeri et al. '704 presents a significant performance advantage for refrigeration systems which experience varying operating parameters, its construction details make it an expensive and difficult assembly to manufacture. Specifically, from a parts count standpoint the valve of Cholkeri et al. '704 includes an inlet and a metering head to provide the first and second flowpaths, a moveable sleeve valving element, a moveable magnetic member to move the sleeve, and a valve outlet in addition to the valve housing and various other minor parts such as bumpers and sealing gaskets. In addition to the high number of parts required to construct the valve of Cholkeri et al. '704, the manufacturing tolerancing of the components also presents a challenge. Specifically, the Cholkeri et al. '704 valve includes a total of five orifices which must be drilled to form the refrigerant flowpaths for the high flow state. To maintain the overall effective orifice size of, for example, 60 thousandths of an inch, the additive tolerance for each of the five holes must be considered and regulated very closely. While the performance of the Cholkeri et al. '704 valve is outstanding, these elements all combine to increase the cost of manufacturing these valves to an undesirably high level.